2009 Fall AGU Abstract

Thomas, S.-M., S. D. Jacobsen, C. R. Bina, J. R. Smyth, and D. J. Frost, IR absorption coefficients for the quantification of water in hydrous ringwoodite, Eos, Transactions of the American Geophysical Union, 90, Fall Meeting Supplement, DI41C-1827, 2009.

DI41C-1827

Raman spectroscopy, combined with the 'Comparator technique', has been developed to determine water contents ranging from a few wt ppm to wt% in glasses and nominally anhydrous minerals including garnets, olivine, and SiO₂ polymorphs (Thomas et al., 2009). The routine is one promising example of quantification tools to determine mineral specific molar absorption coefficients (ε) for IR spectroscopy. Mineral specific absorption coefficients are required because general IR calibrations do not necessarily apply to minerals with water incorporated as hydroxyl point defects. Here we utilize the 'Comparator technique' to provide ε-values for a set of synthetic Fe-free and Fe-bearing (Fo₉₀) ringwoodites, as well as for γ-Mg₂GeO₄. Ringwoodite is considered one of the major phases of the Earth's lower transition zone (520-660 km depth) and the knowledge of its absolute water storage capacity is essential for modeling the Earth's deep water cycle. Samples were synthesized at variable P-T conditions in a multi-anvil press and cover a range of OH contents. Single-crystals were characterized using X-ray diffraction and IR spectroscopy. Mineral specific IR absorption coefficients were calculated from independently determined water contents from Raman spectroscopy. Unpolarized IR spectra of Mg-ringwoodite show broad absorption features in the OH region with band maxima at ∼2350, 2538, 3127, 3172, 3598 and 3688 cm^-1. In the spectra of Fe-bearing ringwoodite and γ-Mg₂GeO₄ the maxima of the main OH band are shifted to 3172 cm^-1 and 3207 cm^-1, respectively. For Mg-ringwoodite with the mean wavenumber (area-weighted average of the peak position) of 3109 cm^-1 an ε-value of 170000 ± 51000 L cm^-2 / mol_H2O was determined. For a Fo₉₀ sample with the mean wavenumber of 3132 cm^-1 the value was calculated to be 123000 ± 37000 L cm^-2 / mol_H2O. The latter two values are in good agreement with the data from the linear calibration of ∼159000 L cm^-2 / mol_H2O and ∼153000 L cm^-2 / mol_H2O, respectively (Libowitzky and Rossman, 1997). However, for another Fe-bearing ringwoodite with the mean wavenumber of 3197 cm^-1 a value of 81000 ± 24000 L cm^-2 / mol_H2O was computed, which deviates from the value of 137000 L cm^-2 / mol_H2O reported by Libowitzky and Rossman (1997). We will further discuss general IR calibrations and the dependence of ε on structure, composition and frequency for the (Mg,Fe)₂SiO₄ polymorphs in the mantle.